Heterogeneous catalysis is inherently a complex subject, and progress toward making it a science rather than an art has required knowledge and techniques in many fields. In recent times physical tools such as electron microscopy and optical and magnetic resonance spectroscopy have seen increasing use and have yielded many interesting new results. This paper is a discussion of Mössbauer spectroscopy, a tool capable of giving new information concerning catalysis. In the first half we review the physical parameters which play a role in the Mössbauer effect and discuss briefly the techniques used in its measurement (the general references for this section are [1] and [2]). In the last half we discuss applications of the effect to studies of adsorption and catalysis with examples from the literature and our own exploratory work.